Bottomhole surveying



Aug. 23, 1966 E. v. WATTS 3,268,859

BOTTOMHOLE SURVEYING Filed July 9, 1962 SOURCE CONTROL\ I RECORDER EUCLID V. WATTS INVENTOR.

V BY Q U United States Patent 3,268,859 BOTTOMHOLE SURVEYING Euclid V. Watts, Houston, Tex, assignor to Mobil Oil Corporation, a corporation of New York Filed July 9, 1962, Ser. No. 208,420 1 Claim. (Cl. 34015.5)

This invent-ion relates to oil well producing operations and more particularly to the determination of the location of the bottom or other selected point along the trajectory followed by a drill bit in forming a well bore.

In the drilling of oil wells, it is often the practice to locate the mouth of the well at a point laterally displaced from the surface projection of the point at which the well is to be bottomed or the point at which fluids are to be produced from an oil-bearing formation. In such cases it is desirable to ascertain with relatively high accuracy the location of such point in the well. In other instances, wells drilled on relatively small leases have been found to have drifted or deviated in the course of drilling in spite of efforts to maintain them within the confines of the lease. As a result, fluids flowing from a producing zone into the well are in fact derived from property adjacent to the lease rather than from the lease itself as ordinarily would be intended.

In either of the foregoing cases it is necessary as a matter of governmental regulation in some cases and as a matter of legal certainty in all cases to be able to have assurance as to the relationship between the surface area of a lease and the actual subsurface point at which production is being derived.

In order to provide such information, instruments have been developed which are lowered into the well bore to provide signals which indicate the trajectory of the bore. Such signals may then be integrated or otherwise totalized to provide a measure of the displacement of the bottom relative to a vertical line passing through the mouth of the well. However, such instruments are expensive and are subject to error in accumulation of totals over long courses as encountered in relatively deep well bores.

In accordance with the present invention, there is provided a method of determining the location of a given point along the course followed by a given well bore which location in general will be unknown even though the length of borehole is known. More particularly, in accordance with the present invention, there is provided a method of surveying a borehole which comprises lowering a sensitive acoustic detector to a point located a predetermined distance from the mouth of said Well bore. Acoustic impulses are successively generated at each of a plurality of points spaced substantial distances from each other in the region of the mouth of the well at known elevations of reference thereto. A signal is generated indicative of the instant of initiation of said acoustic impulses at each of the points, and for each acoustic impulse thus generated electrical signals generated by said detector are recorded along a time scale with reference to the instant of generation thereof.

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

The figure is a diagrammatic representation of a system for carrying out the present invention.

Referring now to the drawings, the boundaries of a producing oil lease are illustrated. For purposes of the present description the surface area of the lease is considered to be the area in which there are located a plurality of Wells including the well 11 at the center thereof. The area 10 may be a quarter section, for example, nine wells are illustrated as being drilled thereon. The mouths of the nine wells are uniformly spaced throughout the area It).

The center well 11 is shown as following a course indicated by the dotted outline 12. While a relatively smooth course is indicated, it is to be understood that in actual practice, the well may extend along a spiral or devious path to the point of completion. Depending upon the many variable factors encountered during drilling, the hole may follow a trajectory down the axis Z and bottom at a point directly and vertically beneath the mouth of the well. In accordance with directional drilling techniques, however, the well may purposely be deviated in order to permit the drilling operation at the surface to be initiated at a location more convenient or accessible or otherwise more desirable than at a point directly above the intended bottomhole location. On occasion the illegal and unauthorized drill-ing of a directional well from a dry lease completed under a distant productive lease has been discovered.

In the present example borehole or well 11 has been indicated as bottomed at a point substantially displaced from the mouth thereof as Well as beyond the bounds of the surface area 10. The displacement may be described in terms of coordinates indicated in the figure as having a total vertical depth z and horizontal displacements x and y.

The present invention provides for an accurate determination of the coordinates x, y and z. The measurements are accomplished by utilizing a detector such as a geophone 20 which is located at the bottom of the well and is supported at the desired point by a cable 21. Cable 21 emerges from the mouth of the borehole 11 and extends to a recorder 22 by way of suitable circuit elements including a reel system (not shown) which is used to lower the geophone or detector 20 into the borehole 11.

At the corners of the area 10 are located shot points SP1, SP2, SP3, and SP4. The shot points in general with be relatively shallow boreholes adapted to receive an explosive charge. The depth at which the explosive charges are located in the shot points SP1-SP4 will be known or otherwise recorded so that they can properly be related to the plane of reference along the vertical axis Z. At SP1, for example, the actual location of the explosive charge may have in general the dimensions x y and 21 with reference to the location of the mouth of the borehole 11. A suitable source control device 23 of the type well-known to those skilled in the art is employed for control of the detonation of an explosive charge in SP1 and simultaneously to generate a time break impulse in known time relation with respect to the instant of impulse generation. Upon detonation an acoustic impulse generated thereby may travel along a path generally indicated by the dashed line 24. The time required for the sound to travel from the explosive at SP1 to the detector 20 may be measured by recording the instant of generation as represented by time break pulse 25 on a record A produced by the recorder 22. The signal as detected by detector 20 then is suitably amplified and recorded by a recording system 22 as a variable amplitude signal on trace 27. The onset 27a of the signal occurs at the instant of arrival of the wave at detector 20. The interval t between pulse 25 and the onset 27a is thus a direct measure of the travel time along path 24. Thus the record A provides an indication of the time required for sound to travel path 24.

Thereafter, record B is produced by detonating an explosive charge in SP2. The time interval t represents the travel time along path 31. Similarly from the record C produced upon detonation of an explosive charge in SP3 the record time t represents the travel time along the path 32. The record D is obtained upon detonation of an explosive charge in SP4. The record time t will indicate the time of travel along the fourth path 33.

If detectors are located in each of the holes in the area during the operations above described, then the multitrace records will provide data for similar record times t t for each of the other holes.

The record times thus produced may be employed for determining the coordinates x, y, z. More particularly, if the depth 2 is known and if there is also known the average velocity V at which sound travels between the points of generation and detection, then the location of the detector 20 can be determined fully from any two of the records AD. If the depth z is not known, but average velocity V is known, then the actual location of the detector 20 can be determined from any three of the four records AD.

If neither the vertical distance z nor the average velocity V are known, then all four records AD will be employed and from calculations hereinafter outlined not only may the location of the detector 20 be determined, but also the average velocity may be determined.

More particularly, employing the notation convention, indicated in the figure as to +X, Y and Z, the following equation may be employed to yield the desired data x, y and 2 directly, as well as V If the well 11 followed a straight line path from top to bottom, the straight line well length L would be expressed as follows:

The coordinates of SP1=x y and Z1 and the coordinates of the other shot points may similarly be described by use of consistent subscripts.

With such notations, the following four equations of the form of Equation 1 may be employed along with data from records AD completely to identify the location of a producing point in well 11:

With four equations and only four unknowns (V x, y, and z), the survey operation may be conducted without any prior information as to velocity V or depth z or the relation between the actual well course and a straight line path between surface and bottom.

In the example illustrated in the drawings, the source locations are at the corners of a quarter section and thus are spaced distances of one-fourth mile. In carrying out the present invention, it is necessary that the source locations be spaced apart distances suificient that the point at which the fluids are extracted from a given formation can be located with required accuracy to make the measuring procedures justifiable. In accordance with the present invention, the source locations are spaced one from the other by distances gauged by the depths to which the boreholes extend. The spacing should not be less than one-tenth the depth of the formation nor greater than a distance equal to the depth of the formation. Preferably, the spacing will be about one-half the depth of the formation. When the operations herein described are conducted in accordance with the foregoing, the point at which a given well actually produces from a given formation can be determined with accuracy of the order of a few feet.

In operation the location of the production point in all of the wells in a given lease may be located by generating waves successively at shot points such as SP1SP4 at the corners of the lease. Detectors at the producing formations in each of the wells as indicated in the drawings facilitate production of records AD. Alternatively of course, the wells can be surveyed one at a time producing but single trace rfisords for each of the necessary shot points.

Where a depth of a producing horizon is known and it is known that wells are bottomed therein, then with known velocity only two single trace records need to be produced for a given well. In any event the above computations can be made either manually from observed times t t from the records or may be made automatically through application of the resultant data to a computer. From such operations the location of any oil wells may be fully described.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claim.

What is claimed is:

The method of determining the coordinates of the location of a subsurface point in a drill hole with respect to the surface opening of said drill hole which comprises:

successively generating a seismic impulse at each of at least four points substantially spaced one from the other in the region of the surface opening of said hole,

sensing the arrival of each said impulse at said subsurface point in said hole,

recording as functions of time relative to the instants of generation of each said impulse a signal representative of the time between the instant of generation of an impulse at each of said four points and the detecting of said impulse at said point in said drill hole, and

combining said signals to produce functions representative of the coordinates of said subsurface point in said borehole in accordance with the following equations:

4 a 4 +(y4 +(z4z) wherein t represents the time between the generation of an impulse at a first of said four points and the detecting of said impulse at said subsurface point in said borehole; t represents the time between the generation of an impulse at a second of said four points and the detecting of said impulse at said subsurface point in said borehole; 1 represents the time between the generation of an impulse at a third of said four points and the detecting of said impulse at said subsurface point in said borehole; and 1 represents the time between the generation of an impulse at a fourth of said four points and the detecting of said impulse at said subsurface point in said borehole; V represents the unknown velocity of acoustic impulses in said formations; where x y Z x y Z2; x y Z3; and x y Z can be positive or negative and respectively represent the known coordinates of said four points, and x, y and z represent the unknown coordinates of said point in said borehole.

References Cited by the Examiner UNITED STATES PATENTS 6/1931 Rosaire 181 12/1941 Slotnick 181 OTHER REFERENCES BENJAMIN A. BORCHELT, Primary Examiner.

CHESTER L. JUSTUS, Examiner.

I. W. MILLS, R. M. SKOLNIK, Assistant Examiners. 

